Several peptide factors present in the thymus gland have been implicated to play important roles in the development and maintenance of immunological competence in man and in animals. The importance of the immune system in the defense against cancer and tumor cells is now widely recognized. In recent years, a few polypeptides shown to be able to stimulate maturation, differentiation and function of T cells have been isolated from bovine thymus. Among them, the acidic peptide thymosin .alpha..sub.1, has been intensively studied. Its structure and activity have been described in U.S. Pat. No. 4,079,127.
Additionally, Goldstein et al. J. of Reticuleondothelial Society 23, 253 (1978) describe partially purified thymosin .beta..sub.3 and .beta..sub.4 as components of the partially purified thymosin fraction 5. Low and Goldstein in Year in Hematology 1978, Siber et al. ed (Plenum Pub. Co. 1978) at p. 281 indicated that partially purified thymosin .beta..sub.3 and .beta..sub.4 induce TdT positive cells in T-cell populations. It has also been reported that thymosin .beta..sub.3 and .beta..sub.4 accelerate the reappearance of TdT positive cells in the thymus following steroid induced immunosuppression [Low, et al. PNAS 78, 1162 (1981); Mol. Cell. Biochem. 41, 49 (1981)], and inhibit the migration of macrophages [Thurman et al., Lymphokines and Thymic Hormones: Their Potential Utilization in Cancer Therapeutics, Goldstein & Chirigos (eds), Raven Press, p. 145 (1981)].
Furthermore in the course of synthesizing thymosin .alpha..sub.1 by solution phase methodology, Wang in U.S. Pat. No. 4,148,788 utilized the protected carboxyl terminal octa-, undeca- and tetradecapeptides of thymosin .alpha..sub.1 as intermediates. These compounds were also deprotected by hydrogenolysis followed by treatment with HF and the resulting free peptides were indicated to have activity in the regulation, differentiation and function of T-cells.